The present invention relates to films, particularly films prepared by blown film extrusion techniques.
Olefin polymers are known to be usefully employed in the preparation of blown films. Particularly suitable olefin polymers include low density polyethylene and linear low density polyethylene. Films from such resins are usefully employed in a number of applications including the preparation of heavy duty shipping bags and grocery sacks. In such applications it is desirable to provide a film having increased modulus which improves the machine handling characteristics of the films. In addition, improved film dart impact resistance and film toughness are also highly desirable properties in order to resist puncture by the contents carried in a heavily loaded bag. Conventionally, polyethylene films meeting the requirements of such end uses have been prepared from blends of low density and high density polyethylene resins. Films prepared from such blends possess high tensile yield strength, but disadvantageously such films have a correspondingly reduced impact strength.
Another aspect of the present invention relates to the use of additives in polyolefin resins. Such additives include pigments and opacity additives to reduce film clarity particularly where surface printing is desired. Additional additives known as anti-blocking aids reduce the "cling" of thin polyolefin films. It would be desirable to provide an olefin polymer film having increased opacity thereby minimizing or eliminating the requirement of pigment loading to produce opacity. An additional desirable achievement would be to provide a film having inherently improved blocking thereby permitting reductions in the level of anti-blocking additives required to be added to such resin in order to achieve easy separation of film surfaces, as for example in opening of bags fabricated from such films.
In U.S. Pat. No. 4,579,912 there are disclosed films prepared from linear low density polyethylene and up to 10% by weight of the total composition of a thermoplastic polymer of an aromatic hydrocarbon. Disadvantageously such films as are disclosed by the above reference tend to be deficient in certain physical properties, particularly dart impact strength and toughness. Moreover, it would be desirable if there were provided a blown film having a better balance of film properties in the machine direction or direction of extrusion versus the transverse direction.